Interpretive Summary: Each plant cell is surrounded by a cell wall, a rigid structure providing mechanical support from external stresses. Underneath the cell wall is plasmamembrane (PM) which provides a boundary between cell wall and the rest of the cell comprised of cytosol and nucleus. PM has numerous physiological and metabolic functions, including the synthesis of cellulose which is a major component of cell wall. The enzyme sucrose synthase (SS) plays a critical role in the utilization of sucrose into diverse functions of a plant. We have shown previously that the loss of this enzyme in a corn mutant leads to both a reduction of starch and brittle cell walls in a developing seed. Although the SS enzyme is most analyzed in corn, all previous data are restricted to only the cytoslo-associated form of the enzyme. The data presented in this manuscript show for the first time in corn that a certain proportion of the enzyme is also associated with PM. This is only the second such report in plants. The first report of an association between the SS enzyme and the PM was described in cotton fiber which is known for its massive amounts of cellulose. These results in corn provide biochemical support to our previous genetic data that SS enzyme is critical in cellulose biosynthesis in plants.

Technical Abstract:
Plasma membrane fractions were isolated from maize (Zea mays L.) Endosperm and etiolated seedlings to investigate the possible membrane location of the sucrose synthase (SS) protein. Endosprems from both 12 and 21 days after pollination (DAP), representing early and mid developmental stages, were used, in addition to etiolated leaf and elongation zones from seedlings. Plasma membrane fractions were isolated from this material using differential centrifugation and aqueous two phase partioning. The plasma membrane enriched fraction obtained was then analyzed for the presence of sucrose synthase using protein blots and activity measurements. Both SS1 and SS2 isozymes, encoded by the Sh1 and Sus1 loci respectively, were detected in the plasma membrane enriched fraction using polyclonal and monoclonal antisera to SS1 and SS2 isozymes. In addition, sucrose synthase activity measurements of endosperm plasma membrane fractions had high levels of specific activity. The sucrose synthase enzyme is tightly associated with the membrane, as shown by Triton X 100 treatment of the plasma membrane enriched fraction. It is noteworthy that the same gene product, from either Sh1 or Sus1, was detectable as both soluble and plasma membrane associated forms.